Activated carbon bed structures are utilized in sorption systems. The carbon beds used in these systems typically have a multiplicity of adsorption sites which capture contaminants and other adsorbates (e.g., water vapor) when contaminated fluid to be filtered is introduced to the bed. Typically there is a porting structure which supplies the contaminated fluid in one direction at one surface of the bed and receives decontaminated fluid at the other end of the bed substantially free of contaminants and other adsorbed species. After the adsorption sites become sufficiently occupied with contaminants and other adsorbed species the bed must be regenerated to remove the captured contaminants and other adsorbed species from the collection sites. This has been accomplished by providing an electric current through the carbon bed which contains conductive material such as iron filings (to conduct the electric current) to heat the bed and raise the bed temperature to an elevated level to facilitate regeneration of the bed. Generally, the electric current is provided to the bed by electrodes that are affixed to the surfaces of the bed where the contaminated fluid enters and the decontaminated fluid exits the bed. Thus, the fluid and electric current flow through the bed in the same direction. Since the electrodes are located in the fluid flow path, they interfere with the fluid flow and increase the fluid flow resistance.
When using electric current to heat the carbon bed it is advantageous to increase the electrical resistance of the bed to cause a greater voltage drop across the bed and thereby allow a lower electric current to be used for the same bed heating rate. To accomplish this, these systems simply increase the height of the bed, which does decrease the electric current flow; however, it also increases the fluid flow resistance. Thus, if the configuration of the carbon bed is modified to alter either the electric current flow or the fluid flow resistance, the other may be adversely affected. This is not a desirable trade-off.
Another problem that persists with using electric current to heat the carbon bed is that there is non-uniform electrical resistance throughout the bed due to the effects of gravity and the migration of smaller size particles to the bed bottom. That is, due to gravity there is more compaction at the bottom of the bed of the carbon and accordingly less electrical resistance due to improved contact between the carbon granules or pellets that comprise the bed. "Fines" of broken pellets or granules collect around points of contact between the larger carbon pellets or granules. This results in undesirable nonuniform electric heating of the bed.
In order to conduct the electric current through the carbon bed between electrodes in these systems, conducting materials such as iron are added to the carbon bed. This, however, introduces problems. The iron occupies space in the carbon bed which reduces the effectiveness of the bed as a decontamination and adsorption medium and the added material rusts or corrodes over time and loses its effectiveness as a conductor.
One prior art system (European Patent Application No. 83304795.4, Rintoul) discloses a method and apparatus for electrically heating columns of activated carbon to reactivate the carbon. This reactivation process is not accomplished in situ, however.